1. Field of the Invention
The present invention relates to advances in electrophotographic image formation, including a toner, a developer, an image forming apparatus and an image forming method.
2. Discussion of the Background
In an electrophotographic image forming apparatus and an electrostatic recording apparatus and the like, a toner is adhered to an electrostatic latent image formed on a photoreceptor to form a toner image thereon, the toner image is transferred onto a transfer material and is fixed thereon with a heat. A full-color image is typically reproduced with four color toners, i.e., a black toner, a yellow toner, a magenta toner and a cyan toner. Each color is developed with each of the toners, which are overlapped on one another and fixed together with heat on a transfer material to form a full-color image thereon.
However, users who are used to seeing prints are not typically satisfied with the image quality produced by a full-color copier yet, and a higher definition and resolution close to photographs and prints thereof are required. It is known that toner having a small particle diameter and a narrow distribution thereof is used to produce high-quality images in electrophotography.
Conventionally, an electrostatic or a magnetic latent image is visualized by a toner. The toner for use in developing the electrostatic latent image is a colored particle formed of a binder resin including a colorant, a charge controlling agent and other additives. The toner is mostly prepared by a pulverization method and a polymerization method.
The pulverization method includes kneading a colorant, a charge controlling agent, offset inhibitor and the like in a thermoplastic resin upon application of heat, uniformly dispersing them therein to prepare a mixed composition, and pulverizing and classifying the mixed composition to prepare a toner. The pulverization method can prepare a toner having good properties to some extent, however, materials therefor are limited. For example, the mixed composition prepared by kneading upon application of heat should be pulverized and classified by an economically usable pulverizer. Therefore, the mixed composition should be sufficiently blendable.
The pulverized toner tends to have a wide particle diameter distribution, and for example, fine particles having a diameter not greater than 5 μm and coarse particles having a diameter not less than 20 μm have to be removed to produce images having good image resolution and tone reproduction, and therefore yield decreases. In addition, it is difficult to uniformly disperse the colorant, charge controlling agent and the like in the thermoplastic resin by the pulverization method. Nonuniform dispersion thereof adversely affects fluidity, developability and durability of the resultant toner and image quality produced thereby.
Japanese Laid-Open Patent Publication No. 9-43909 discloses a suspension polymerization method of preparing a toner. However, the toner prepared thereby has a spherical shape but has poor cleanability. Since there is less residual toner after transferred when an image having a low image area is developed or transferred, the poor cleanability is not a serious problem. However, when an image having a high image area such as a photograph image is developed or transferred, untransferred toner remains on a photoreceptor as a residual toner after transferred, and which causes background fouling of images produced thereby when accumulated. In addition, the toner contaminates a charging roller charging the photoreceptor while contacting the photoreceptor and impairs the original chargeability thereof. Further, the toner does not have sufficient low-temperature fixability and much energy us consumed to fix the toner.
Japanese Patent No. 2537503 discloses a method of preparing an amorphous toner particle by assembling a particulate resin prepared by an emulsifying polymerization method. However, a large amount of a detergent remains not only on the toner particle but also therein, and impairs atmospheric charge stability and widens charge quantity distribution of the toner, resulting in background fouling of images produced thereby. In addition, the remaining detergent contaminates a photoreceptor, a charging roller and a developing roller and impairs the original chargeability thereof.
In a fixing process by contact heating using a heating member such as a heat roller, releasability of a toner particle from the heating member (hereinafter referred to as offset resistance) is required. The offset resistance can be improved by making a release agent present on the surface of the toner particle. Japanese Laid-Open Patent Publications Nos. 2000-292973 and 2000-292978 disclose a method of improving the offset resistance by not only including a particulate resin in the toner particle but also unevenly distributing the particulate resin on the surface thereof. However, a lowest fixable temperature increases and low-temperature fixability, i.e., energy saving fixability is insufficient.
Further, the method of preparing an amorphous toner particle by assembling a particulate resin prepared by an emulsifying polymerization method has the following problems. Namely, when a particulate release agent is assembled with a toner particle to improve the offset resistance thereof, the particulate release agent is taken therein, resulting in insufficient improvement of the offset resistance. Since a particulate rein, a particulate release agent and a particulate colorant are randomly fusion bonded to form a toner particle, compositions (component content ratios) and molecular weights of resins fluctuate. Therefore, surface properties of the toner particles are different from one another and images having good quality cannot be produced for long periods. Further, the particulate resin unevenly distributed on the surface of a toner impairs low-temperature fixability thereof.
A solution suspension method is known as a method of preparing a toner. The method has the advantage of being able to use a polyester resin capable of being fixed at a low temperature. However, since a polymer material is included in a process of dissolving or dispersing a low-temperature fixable resin and a colorant in a solvent in this method, the resultant liquid viscosity increases and the productivity reduces. In the solution suspension method, Japanese Laid-Open Patent Publication No. 9-15903 discloses a method of improving cleanability of a toner by making the shape of a toner spherical and having the surface thereof concave and convex. However, when a wax is included in such a toner as a release agent, the wax and a pigment are less dispersed therein than in a kneaded and pulverized toner and the wax is granulated in a solvent. Therefore, the resultant toner does not have sufficient releasability.
The suspension polymerization method, emulsifying polymerization method and solution suspension method typically use a styrene acrylic resin, and has difficulty in controlling diameter, a distribution thereof and a shape when using a polyester resin. In addition, the lowest fixable temperature has a limit.
Japanese Laid-Open Patent Publication No. 11-133667 discloses a method of using a urea-modified polyester resin for the purpose of improving thermostable preservability and low-temperature fixability. However, the resultant toner does not have sufficient atmospheric charge stability.
In the electrophotographic field, higher quality images are studied from various angles, and particularly it is recognized that the smaller diameter and conglobation of a toner are highly effective. However, the smaller the diameter of the toner, the lower the transferability and fixability thereof, resulting in production of images having poor quality. Japanese Laid-Open Patent Publication No. 9-258474 discloses a method of conglobating a toner to improve transferability thereof. In this surroundings, color copiers and color printers are required to produce images at a higher speed.
To produce images at a higher speed, a tandem method can be used as disclosed in Japanese Laid-Open Patent Publication No. 5-341617. The tandem method is a method of producing a full-color image on a transfer paper by sequentially overlying each of images produced by plural image forming units thereon. The tandem-type full-color image forming apparatus can use a variety of transfer papers and produces high-quality full-color images at a higher speed than the other types of full-color image forming apparatus. An attempt to use a spherical toner is also made. However, a higher fixability is required to produce images at a higher speed, and the spherical toner does not have good low-temperature fixability.
A toner is required not to agglutinate and have no or less deterioration of chargeability, fluidity, transferability and fixability even in an environment of high temperature and high humidity, and that of low temperature and low humidity when stored and transported. However, such a spherical toner is not available.
A heat and pressure fixing method of directly contacting a fixing member such as a fixing roller and a fixing belt to an unfixed image to fix the image on an image bearer such as a paper upon application of pressure and heat is preferably used because its good heat efficiency, simple structure and low production cost.
Particularly, a fixer using a belt-shaped heating medium (hereinafter referred to as a fixing belt) is widely used recently.
The fixing belt can contact the surface of a toner for a long time when fixing and can fix the toner at a lower temperature, and at the same time, toner is melted so much that fusion bonding thereof on the surface of the belt, i.e. an offset phenomenon tends to occur. Particularly, a color toner needs a moderate gloss to appeal sufficient transparency and high grade feeling, and a binder resin needs to have a sharp molecular weight and a sharp-melt property. Therefore, the color toner is melt so much that the offset phenomenon tends to occur.
The fixing belt is mostly an endless belt or a belt with an end formed of a thermostable resin, and mechanical durability thereof should be improved. An external additive and other components of a toner adhere to the belt-shaped heating medium, and which causes an abrasion or a damage thereof, resulting in occurrence of hot offset and a rift of the belt.
Various suggestions have been made to solve the above-mentioned problems as for both of the fixer and toner. Studies and suggestions of only the fixer cannot become an essential solution.
A release agent such as a wax is included in a toner to prevent offset when fixed. However, when a property of the wax and a dispersion status thereof in the toner is unsuitable, the wax is released and from the surface of the toner and exuded thereon after used for long periods in a developing unit. When such a toner is used in a two-component developer, the carrier is contaminated, resulting in deterioration of chargeability of the toner. When used in one-component developer, the wax is fusion bonded to a developing roller and a blade forming a thin layer of the toner, resulting in nonuniform development with the toner. Therefore, a wax is preferably included in a toner in a small amount.
A toner prepared by conventional kneading and pulverization typically has an amorphous shape, a broad particle diameter distribution, a low fluidity, a low transferability, a high fixable temperature, nonuniform charge quantity and a low charged stability. Particularly, the toner has a crack due to the pulverization at an interface of a release agent (wax) and the release agent is present on the surface thereof in a large amount. Therefore, the toner has a sufficient releasability, but the release agent tends to adhere to a carrier, a photoreceptor and a blade.
Japanese Patents Nos. 3413024 and 3397661 and Japanese Laid-Open Patent Publication No. 2002-351143 disclose a method of controlling fixability of a toner by controlling a molecular weight distribution of tetrahydrofuran(THF)-soluble components or a ratio of tetrahydrofuran(THF)-insoluble components. However, particularly a toner including a large amount of crosslinked components does not have satisfactory hot offset resistance, cold offset resistance, fixability and glossiness without producing images having background fouling and contaminating a carrier only by controlling the THF-soluble components.
Because of these reasons, a need exists for a toner having sufficient low-temperature fixability, offset resistance, friction resistance, glossiness without contaminating a fixer and an image even after producing tens of thousands of images.